Arenaviruses are distributed in two distinct subgroups defined geographically and serologically. Old World arenaviruses are lymphocytic choriomeningitis virus (LCMV) found worldwide, and the African arenaviruses Lassa and Mozambique. New World arenaviruses include 8 to 10 antigenically related viruses found in South America and the Caribbean which collectively form the Tacaribe complex. Members of both groups exist in Nature as persistent infections in rodents, and several cause severe acute disease in man. I have previously studied the polypeptide composition of the prototype arenavirus, LCMV, and have raised a library of monoclonal antibodies to the viral polypeptides. The overall objectives of the present study are: 1) to conduct further detailed studies on the polypeptides of LCMV, their structure and expression in infected cells; 2) to assess the molecular basis of antigenic relationships by mapping cross reactive and type specific antigens defined by monoclonal antibodies on the proteins of arenaviruses; and 3) to study antigen expression and specific antigen reactive T-lymphocyte populations in LCMV infected mice. These studies will provide basic information necessary to interpret the structure and evoluation of arenaviruses and to understand arenavirus disease.